Electronic, bonding, linear, and nonlinear optical properties of Na2MGe2Q6 (M=Cd, Zn, Hg; Q=S, Se), Na2ZnSi2S6, and Na2ZnSn2S6 two metal-mixed chalcogenide compounds: Insights from an ab initio study

In this study, in order to understand the origins of the bonding, electronic, and optical properties of Na2MGe2Q6 (M = Cd, Zn, Hg; Q = S, Se), Na2ZnSi2S6, and Na2ZnSn2S6 compounds, we conducted first principles calculations within the density functional theory framework. We analyzed the sensitivity of replacing cations and anions with different electronegativity, which were rationalized in terms of the electronic structure and the contributions of different orbitals. Our calculations yielded lattice parameters, band gap, dipole moments, and second harmonic components that generally agreed well with the available experimental data. In addition, the electron localization function and atom-in-molecule topological formalisms were used to obtain further insights into the bonding properties. The calculations demonstrated the good concordance between the nature of the analyzed electronic domains and the response to second harmonic generation for the compounds studied. Moreover, the optical properties of these materials were found to be highly sensitive to the combined effects of the [Si/Ge/SnQ4] and [MQ4] units.